#include <wchar.h>
#include <stdio.h>

enum { WCHAR_BUF = 128 };
const wchar_t ERROR_PREFIX[7] = L"Error: ";
void func(void){
    const size_t prefix_len = wcslen(ERROR_PREFIX);
    wchar_t error_msg[WCHAR_BUF];
    wcscpy(error_msg,ERROR_PREFIX);
    fgetws(error_msg + prefix_len,WCHAR_BUF - prefix_len,stdin);
    /* ... */
}